Generally, it’s considered bad leadership for a chief executive to castigate one of his subordinates in public. I suspect Trump did it because he wants Sessions to resign rather than having to fire him.
Castigating your subordinates in an interview with a media outlet that has worked ceaselessly to undermine you -- and which you refer to regularly as "the Failing New York Times" in your Twitter posts -- sounds more like a staged "castigation" that nobody should be taking seriously anyway.